The present invention relates to a switching power supply unit and a driving method thereof, and in particular to a synchronous rectification switching power supply unit that uses switch elements in an output rectifier and a driving method thereof.
Conventionally, a so-called DC-to-DC converter is known as a switching power supply unit. A representative DC-to-DC converter converts a direct current (DC) into an alternating current (AC) by using a switching circuit, transforms (steps up/down) the AC by using a transformer, and converts the resulting AC into a DC by using an output circuit, thereby obtaining a DC output having a voltage different from the input voltage.
In some cases, an output rectifier used in a DC-to-DC converter employs a switch element such as a transistor for control in synchronization with an input switching circuit. ADC-to-DC converter having such an output rectifier is generally called a synchronous rectification switching power supply unit.
FIG. 1 is a circuit diagram showing a general synchronous rectification switching power supply unit.
As shown in FIG. 1, a synchronous rectification switching power supply unit includes: a transformer 2 where a primary winding is connected to a positive terminal of a DC input supply 1; a first transistor 3 connected between a negative terminal of the DC input supply 1 and the primary winding of the transformer 2; an input capacitor 4 connected across the terminals of the DC input supply 1; an output rectifier 7 having a second transistor 5 and a third transistor 6, the output rectifier rectifying waveforms that appear at a secondary winding of the transformer 2; an output smoothing section 10 having a choke coil 8 and a smoothing capacitor 9, the output smoothing section smoothing the output of the output rectifier 7; a control circuit 11 for generating a control signal C based on the output voltage Vo; timing adjusters 12 through 14 for respectively providing the control signal C with predetermined delays; a buffer 15 for generating a first gate signal Vg1 supplied to the gate of the first transistor 3 based on the output of the timing adjuster 12; a buffer 16 for generating a second gate signal Vg2 supplied to the gate of the second transistor 5 based on the output of the timing adjuster 13; and an inverter 17 for generating a third gate signal Vg3 supplied to the gate of the third transistor 6 based on the output of the timing adjuster 14. The output of the output smoothing section 10 is connected to a load 18 to be driven.
FIG. 2 is a timing chart showing a conventional art driving method in the aforementioned synchronous rectification switching power supply unit.
In a synchronous rectification switching power supply unit of this kind, the first transistor 3 and the third transistor 6 alternately repeats turning on and turning off. The basic operation is to turn ON the second transistor 5 while the first transistor 3 is ON.
As shown in FIG. 2, in the conventional driving method, to shift the first transistor 3 from OFF to ON and shift the third transistor 6 from ON to OFF, the third gate signal Vg3 is driven low to turn OFF the third transistor 6 (time t0), the first gate signal Vg1 is driven high to turn ON the first transistor 3 (time t1), and finally the second gate signal Vg2 is driven high to turn ON the second transistor 5 (time t2) To shift the first transistor 3 from ON to OFF and shift the third transistor 6 from OFF to ON, the second gate signal Vg2 is driven low to turn OFF the second transistor 5 (time t3), the first gate signal Vg1 is driven low to turn OFF the first transistor 3 (time t4), and finally the third gate signal Vg3 is driven high to turn ON the third transistor 6 (time t5).
In this way, delay amount of each of the timing adjusters 12 through 14 is set so that the timings of the first to third gate signals Vg1 through Vg3 are provided as mentioned earlier. By setting the delay amount of the timing adjusters 12 through 14 and changing the first to third gate signals Vg1 through Vg3 with the timings shown in FIG. 2, it is possible to prevent the first transistor 3 and the third transistor 6 from turning ON simultaneously and causing a through current to flow.
FIG. 7 is a circuit diagram showing a general synchronous rectification switching power supply unit where current mode control is performed.
As shown in FIG. 7, a synchronous rectification switching power supply unit includes: a transformer 102 where a primary winding is connected to a positive terminal of a DC input supply 101; a first transistor 103 and a resistor 120 connected between a negative terminal of the DC input supply 101 and the primary winding of the transformer 102; an input capacitor 104 connected across the terminals of the DC input supply 101; an output rectifier 107 having a second transistor 105 and a third transistor 106, the output rectifier rectifying waveforms that appear at a secondary winding of the transformer 102; an output smoothing section 110 having a choke coil 108 and a smoothing capacitor 109, the output smoothing section smoothing the output of the output rectifier 107; a control circuit 111 for generating a control signal C based on the output voltage Vo; timing adjusters 112 through 114 for respectively providing the control signal C with predetermined delays; a buffer 115 for generating a first gate signal Vg1 supplied to the gate of the first transistor 103 based on the output of the timing adjuster 112; a buffer 116 for generating a second gate signal Vg2 supplied to the gate of the second transistor 105 based on the output of the timing adjuster 113, and an inverter 117 for generating a third gate signal Vg3 supplied to the gate of the third transistor 106 based on the output of the timing adjuster 114. The output of the output smoothing section 110 is connected to a load 118 to be driven.
The resistor 120 is used to extract a current iFET1 flowing the first transistor 103 as a voltage value. The extracted voltage value is supplied to the control circuit 111 as a current signal S.
FIG. 8 is a timing chart showing a method for generating a control signal C.
As shown in FIG. 8, in the control circuit 111, the output voltage Vo is compared with the current signal S and the control signal C is asserted in response to an internal clock. The control signal C is negated with the timing the value of the current signal S has reached the output voltage Vo. Accordingly, the duty cycle of the control signal C is controlled based on the output voltage Vo and the current signal S. A method for setting the duty cycle of the control signal C based on the comparison between the output voltage Vo and the current signal S is generally called xe2x80x9ccurrent mode control.xe2x80x9d
In the synchronous rectification switching power supply unit shown in FIG. 1 and driven by the driving method shown in FIG. 2, in case the load 18 is light and the output current Io is small, a choke current iL may be inverted in a period the first transistor 3 is OFF (from time t5 to next time T0), as shown in FIG. 2. In this case, the inverted current flows via the third transistor 6 that is ON. When the third transistor 6 turns OFF at time t0, the current flow is interrupted and appears as a fly-back voltage across the third transistor 6, as shown in FIG. 2.
Such a fly-back voltage depends on the energy accumulated in the choke coil 8 and may exceed the withstand voltage of the third transistor 6 thus damaging an element. In order to prevent this, it was necessary to use a transistor having a sufficiently high withstand voltage as the third transistor 6 in the related art.
The Japanese Patent Publication No. H11-289760 shows a technology to suppress an inverted current by detecting or predicting the occurrence of an inverted current as an approach to prevent occurrence of a fly-back voltage.
However, considering the accuracy and temperature characteristics of elements used, it is difficult to correctly detect occurrence of the inverted current. Even in case a preset value is used to predict occurrence of the inverted current, providing an ample margin considering the accuracy and temperature characteristics of elements used increases the period both of the second transistor 5 and the third transistor 6 are OFF, called a dead time, thus increasing the loss. Moreover, a circuit is necessary to detect or predict occurrence of an inverted current thus increasing the number of elements.
Further, in the method for generating the control signal C shown in FIG. 8 in the synchronous rectification switching power supply unit shown in FIG. 7, while in case the load 118 is heavy and the output current Io is large, comparison is correctly made between the output voltage Vo and the current signal S as shown in FIG. 8, in case the load 118 is light and the output current Io is small, a spike current caused by a discharge current to the parasitic capacity of the third transistor 106 or a recovery current for a parasitic diode of the third transistor 106 may exceed the actual peak value of the current signal S. In such a case, the control signal C is negated in response to the spike current and a correct duty cycle is not obtained. In particular, a synchronous rectification switching power supply unit often uses a plurality of transistors connected in parallel as the second transistor and the third transistor 106 in order to reduce the loss in the output rectifier 107. In such a case, the spike current is more noticeable.
In order to solve this problem, a low-pass filter may be used to remove a spike waveform from the current signal S. However, this method increases the number of elements and distorts the waveform of the current signal S thus preventing correct control by the control circuit 111.
Thus, the object of the invention is to provide a switching power supply unit and that can effectively prevent occurrence of a fly-back voltage without increasing the number of elements its driving method.
Further, the object of the invention is to provide a switching power supply unit that performs current mode control, the switching power supply unit effectively preventing occurrence of a spike waveform of a current signal without increasing the number of elements, and its driving method.
The object of the invention is attained by a switching power supply unit including: a switch circuit equipped with at least a first switch, the switch circuit converting a DC input into an AC; a transformer for transforming the AC; an output rectifier equipped with at least a second switch serially connected to the transformer and a third switch connected in parallel to the transformer, the output rectifier rectifying the output of the transformer; and a controller for controlling ON/OFF of the first to third switch, wherein the controller turns ON the second switch before turning ON the third switch and turning ON the first switch.
The object of the invention is also attained by a switching power supply unit including: a switch circuit equipped with at least a first switch, the switch circuit converting a DC input into an AC, a transformer for transforming the AC; an output rectifier equipped with at least a second switch serially connected to the transformer and a third switch connected in parallel to the transformer, the output rectifier rectifying the output of the transformer; an output smoothing section equipped with at least a choke coil serially connected to the transformer and a smoothing capacitor connected in parallel to the transformer, the output smoothing section smoothing the output of the output rectifier; and a controller for controlling ON/OFF of the first to third switch, wherein the controller turns ON the second switch then turns OFF the third switch while the inverted current from the choke coil is flowing into the third switch.
Preferably, the controller controls ON/OFF of the first to third switches via voltage mode control.
The object of the invention is also attained by a driving method for a switching power supply unit including a switch circuit equipped with at least a first switch, the switch circuit converting a DC input into an AC, a transformer for transforming the AC, and an output rectifier equipped with at least a second switch serially connected to the transformer and a third switch connected in parallel to the transformer, the output rectifier rectifying the output of the transformer, the driving method including the steps of: turning ON the second switch; turning OFF the third switch; and turning ON the first switch.
The object of the invention is also attained by a driving method for a switching power supply unit including a switch circuit equipped with at least a first switch, the switch circuit converting a DC input into an AC, a transformer for transforming the AC, an output rectifier equipped with at least a second switch serially connected to the transformer and a third switch connected in parallel to the transformer, the output rectifier rectifying the output of the transformer, and an output smoothing section equipped with at least a choke coil serially connected to the transformer and a smoothing capacitor connected in parallel to the transformer, the output smoothing section smoothing the output of the output rectifier, the driving method including the steps of: lowering the voltage across the first switch by feeding the inverted current from the choke coil into the transformer; and then turning ON the first switch.
According to the switching power supply unit and its driving method of the invention as described earlier, a fly-back voltage does not occur across the third switch element thus preventing damage to the elements as well as eliminating the need for using a switch having a high withstand voltage as the third switch. Further, when the first switch turns ON the voltage across the first switch is lowered so that it is possible to reduce a switching loss caused by the first switch.
The object of the invention is attained by a switching power supply unit including: a switch circuit equipped with at least a first switch, the switch circuit converting a DC input into an AC; a transformer for transforming the AC; an output rectifier equipped with at least a second switch serially connected to the transformer and a third switch connected in parallel to the transformer, the output rectifier rectifying the output of the transformer; and a controller for controlling ON/OFF of the first to third switch via current mode control, wherein the controller turns ON the second switch before turning ON the third switch and turning ON the first switch.
The object of the invention is also attained by a switching power supply unit including: a switch circuit equipped with at least a first switch, the switch circuit converting a DC input into an AC; a transformer for transforming the AC; an output rectifier equipped with at least a second switch serially connected to the transformer and a third switch connected in parallel to the transformer, the output rectifier rectifying the output of the transformer; an output smoothing section equipped with at least a choke coil serially connected to the transformer and a smoothing capacitor connected in parallel to the transformer, the output smoothing section smoothing the output of the output rectifier; and a controller for controlling ON/OFF of the first to third switch via current mode control, wherein the controller turns ON the second switch then turns OFF the third switch while the inverted current from the choke coil is flowing into the third switch.
Preferably, the current mode control uses at least the information indicating the volume of a current flowing in the transformer and the information indicating the output voltage value of the output smoothing section to control ON/OFF of the first to third switches.
The object of the invention is also attained by a driving method for a switching power supply unit including a switch circuit equipped with at least a first switch, the switch circuit converting a DC input into an AC, a transformer for transforming the AC, an output rectifier equipped with at least a second switch serially connected to the transformer and a third switch connected in parallel to the transformer, the output rectifier rectifying the output of the transformer, and an output smoothing section equipped with at least a choke coil serially connected to the transformer and a smoothing capacitor connected in parallel to the transformer, the output smoothing section smoothing the output of the output rectifier, the driving method including the steps of: generating a control signal by using at least the information about the volume of a current flowing in the transformer and the information about the output voltage value of the output smoothing section to generate a control signal; and, based on the control signal, turning ON the second switch, turning OFF the third switch, and then turning ON the first switch.
According to the switching power supply unit and its driving method of the invention as described earlier, it is possible to correctly control ON/OFF of the first to third switch via current mode control. Moreover, a fly-back voltage does not occur across the third switch element thus preventing damage to the elements as well as eliminating the need for using a switch having a high withstand voltage as the third switch. Further, when the first switch turns ON the voltage across the first switch is lowered so that it is possible to reduce a switching loss caused by the first switch.